Conventionally, a turbo code is available as an error correcting/encoding scheme and used by the 3GPP for standardization. Compared to other error correcting schemes, this turbo code is characterized by its ability to obtain an error rate characteristic of extremely high quality.
An example of a transmission apparatus using a conventional turbo code is shown in FIG. 1. The transmission apparatus 10 shown in FIG. 1 is provided with a turbo encoding section 11, a parallel/serial (hereinafter referred to as “P/S”) conversion section 12, a spreading section 13, a radio transmission section 14 and an antenna 15.
A turbo encoding section 11 receives a transmission signal, converts the signal to a turbo code, generates systematic bit data and parity bit data, and gives those data to the P/S conversion section 12. The P/S conversion section 12 receives the systematic bit data and parity bit data from the turbo encoding section 11, carries out P/S modulation, generates a serial modulated signal and gives the signal to the spreading section 13.
The spreading section 13 carries out spreading processing on the modulated signal from the P/S conversion section 12, generated a spread signal and gives the spread signal to the radio transmission section 14. Here, according to the 3GPP (Third Generation Partneship Project) standardization scheme, the spreading section 13 uses the same spreading code for systematic bit data and parity bit data of the modulated signal, and uses the same spreading factor and the same number of spreading codes assigned. The radio transmission section 14 receives the spread signal from the spreading section 13 and sends the signal through the antenna 15.
Here, as a conventional CDMA transmission apparatus, there is one provided with a section that spreads one user signal with a plurality of spread signals assigned thereto to improve the error rate characteristic.
Furthermore, the conventional CDMA transmission apparatus is also provided with a data addition section for converting an input data string with variable bits to a data string XA with a predetermined number of bits, a first encoding section for encoding the data string XA, a second encoding section for encoding a data string XA, after interleaving and two puncturing sections for puncturing the outputs of the first and second encoding sections.
However, the conventional CDMA transmission apparatus has a problem that when a plurality of spread signals is assigned to one user signal for spreading to improve its error rate characteristic, the number of user signals that can be multiplexed is reduced, and therefore the frequency utilization efficiency is reduced and when the spreading factor is increased, the error rate characteristic may be improved but the transmission efficiency decreases. That is, the conventional CDMA transmission apparatus has a problem that it is difficult to make improvement of the error rate characteristic compatible with improvement of the transmission efficiency.